I get friend invites from people who were at my school, not in my year, that I've never spoken to.
So I guess some people just add everyone.
My best friend has attempted to add everyone on our course (~120) on FB and also got their uni emails from the uni address book and added them all to his phone.
I never add anyone, but accept requests from people on my course/from work. I currently have 234 friends, most of whom I haven't talked to since leaving school in 2010.